Thermoplastic foams made from alkenyl aromatic polymers (e.g, polystyrene) or polyolefin polymers (e.g. polyethylene) have found extensive use, particularly as insulating materials. Generally, insulating foams are produced in thicknesses greater than about one-half inch. The insulating value of such foams is measured in terms of heat conduction resistance or R-value, per one inch of foam thickness. Adequate insulating foams typically have R-values of about 4.0 per inch or greater. It is also desirable for such insulating foams to be dimensionally stable.
These and other polymer foams are commonly made by using a blowing agent with molten resin under pressure and, after thorough mixing, extruding the combination through an appropriate die into a lower pressure atmosphere.
In the past, physical blowing agents widely used for making foams were chlorofluorocarbons, which, because of their high ozone depletion potential (ODP), were subsequently replaced with hydrochlorofluorocarbons (HCFCs). The use of HCFC blowing agents, however, will likely be banned in the near term, again because of high ODP. Some of these blowing agents can be replaced with hydrofluorocarbons (HFCs), which have zero ODP, so as to minimize damage to the ozone layer. Furthermore, HFCs have a thermal conductivity lower than most polymers or blowing agents, so HFC residuals in a polymeric foam can lower the foam's thermal conductivity.
Presently, physical blowing agents more commonly used for making thermoplastic polymer foams such as alkenyl aromatic polymer (e.g, polystyrene) or polyolefin polymer (e.g. polyethylene) foams, are hydrocarbons, chlorinated hydrocarbons, hydrochlorofluorocarbons, or combinations thereof. Hydrocarbons with three or more carbons atoms are considered volatile organic compounds (VOCs) that can lead to the formation of smog. Halogenated hydrocarbons often are either VOCs or have high ODP or are hazardous air pollutants (HAPs), and, at times, may fall into more than one of these categories. Therefore, the use of such blowing agents for preparing polymeric foams is not preferred environmentally and imposes many limitations on the manufacturing process, thus complicating and significantly increasing the cost of manufacturing.
Methyl formate is classified as a non-VOC (Federal Register, Volume 69, Number 228, Nov. 29, 2004), is non-HAP, and has zero ODP. U.S. Pat. No. 6,753,357 describes the use of methyl formate to produce stable, rigid isocyanate/polyol based polyurethane foams. It is noted, however, that such polyurethane foams are thermoset, so as to be made via a cross-linking and curing process. The dimensional stability or instability imparted to the final polyurethane foam product by the nature of the blowing agent therefore is quite different than in the case of melt processed thermoplastic polymer foams.
Therefore, a need exists for blowing agents employing methyl formate as one of the components of the blowing agent blend to produce stable thermoplastic foams without compromising the product quality in terms of appearance, mechanical or compressive strength and insulation value, and that enable a cost-effective and versatile manufacturing process.